Here are the identified mistakes and their corrections:

1. **Mistake:** "an"
**Correction:** "a"
**Corrected Sentence:** Viet Nam is a multicultural country with 54 ethnic groups.

2. **Mistake:** "are talking"
**Correction:** "were talking"
**Corrected Sentence:** When the bus arrived, we missed it because we were talking.

3. **Mistake:** No error in this sentence. It is grammatically correct.

4. **Mistake:** "tries"
**Correction:** "tried"
**Corrected Sentence:** Although she was old, she tried to climb up Yen Tu Mountain.

5. **Mistake:** "It's is"
**Correction:** "It's" or "It is"
**Corrected Sentence:** It's important to preserve our traditional festivals for future generations. (Alternatively: It is important...)

So the fixed sentences are:

1. Viet Nam is a multicultural country with 54 ethnic groups.
2. When the bus arrived, we missed it because we were talking.
3. (No correction needed)
4. Although she was old, she tried to climb up Yen Tu Mountain.
5. It's important to preserve our traditional festivals for future generations.